Fix ibv_port_query() usnic extension usage

* Older versions of libusnic_verbs actually return 0 when querying for
  an unknown port.  So also check for a magic ID in the returned data
  to *really* know if the usnic extensions are supported.
* Use a union (in the common_verbs area) and memcpy (in the btl) to
  avoid undefined C type aliasing behavior.
* Ensure to memset the function table to 0 if the usnic extensions
  are not supported.

Submitted by Jeff Squyres, reviewed by Dave Goodell.

cmr=v1.7.5:reviewer=ompi-rm1.7